Binuclear aluminum complexes supported by linked bis(β-diketiminate) ligands for ring-opening polymerization of cyclic esters

Binuclear aluminum alkyl complexes 2a–4g supported by linked bis(β-diketiminate) ligands were synthesized via the reaction of AlEt3 or AlMe3 and the corresponding proligand in a 2:1 molar ratio with moderate yields. The isolated complexes were well-characterized by 1H-NMR, 13C-NMR and elemental analysis. The binuclear nature of aluminum complex 2b was further confirmed by an X-ray diffraction study. All complexes 2a–4g could efficiently initiate the ring-opening polymerization (ROP) of ε-caprolactone in toluene. The substituents at the aromatic rings and the linker unit in the auxiliary ligands exerted significant influence on the catalytic behavior of the investigated aluminum complexes. Complex 4g (R1 = R2 = Cl) containing propylenyl bridging unit exhibited the highest catalytic activity among these complexes, which might be attributed to the increased electrophilicity of the metal center as well as more opened coordination sphere. The molecular weights of obtained poly(ε-caprolactone)s deviating considerably from the theoretical values indicated that the ROP of ε-caprolactone by complexes 2a–4g was not well-controlled, which was also judged from the broad molecular weight distributions (MWD = 1.47–2.47) of produced poly(ε-caprolactone)s. These complexes proved to be inactive toward the polymerization of rac-lactide alone. In the presence of alcohol the polymerization occurred, which was actually initiated by the decomposition species of the aluminum complex upon the treatment with isopropanol.